Carburetors of this kind are generally known and are usually utilized in motor chain saws. In the cold start position, the choke flap is closed and the throttle flap is partially open; whereas, in the full-throttle position, the choke flap as well as the throttle flap are fully open. For a warm running engine, the choke flap is always held in its out-of-service position which can be achieved by means of a manual positioning member or a partially-automatic positioning member.
Membrane carburetors of this kind are as a rule tuned in their idle position and in their full-throttle position for adjusting the air/fuel mixture. This can lead to the condition that a relatively lean mixture is made available by the membrane carburetor in the part-load region of the internal combustion engine and this leads to a poor running performance of the engine. In two-stroke engines, the lubrication occurs via the two-stroke oil mixed in with the fuel. In such engines, a mixture which is too lean can furthermore lead to engine damage with a complete loss of the engine resulting therefrom. Lean mixtures further lead to temperature increases which can lead to temperature damage in the engine.
Structural measures such as additional bores or the like can be used to tune the membrane carburetor in the part-load range. However, this leads to an overenrichment of the mixture when running down from a high rpm to a lower rpm whereby the engine can die.